Field of the Invention
The present invention relates to an LED (light emitting diode) driving method, especially to a method for minimizing LED flicker of an LED driver system.
Description of the Related Art
Please refer to FIG. 1, which illustrates a circuit diagram of a prior art LED driver circuit. As illustrated in FIG. 1, the prior art LED driver circuit includes a bridge rectifier 10, a PFC constant average current converter 11, a capacitor 12, an LED module 13, an NMOS (n type metal oxide semiconductor) transistor 14, an amplifier 15, and a current-setting resistor 16.
The bridge rectifier 10 is used to generate a full-wave-rectified voltage VIN according to an AC power VAC.
The PFC (power factor correction) constant average current converter 11 is coupled with the bridge rectifier 10 to make an input current IIN in phase with the full-wave-rectified voltage VIN, and generate an output voltage VOUT and an output current IOUT, which are both in phase with the full-wave-rectified voltage VIN, and the output current IOUT has a constant average value.
The capacitor 12 is coupled between the PFC constant average current converter 11 and a ground for bypassing a ripple current IC.
The LED module 13 has a first end coupled with the PFC constant average current converter 11 and a second end coupled with the NMOS transistor 14 to let a load current ILED flow through, wherein ILED=IOUT−IC.
The NMOS transistor 14, the amplifier 15, and the current-setting resistor 16 form a current regulator, wherein the NMOS transistor 14 has a drain coupled with the second end of the LED module 13, a gate coupled with the amplifier 15, and a source coupled with the current-setting resistor 16; the amplifier 15 has a positive input end coupled with a constant reference voltage VREF, a negative input end coupled with the source of the NMOS transistor 14, and an output end coupled with the gate of the NMOS transistor 14; and the current-setting resistor 16 is coupled between the source of the NMOS transistor 14 and the ground.
The current regulator is designed to regulate a current flowing through the current-setting resistor 16 to a constant current of VREF/(the resistance of the current-setting resistor 16). As the current-setting resistor 16 is in the same current path with the LED module 13, therefore, the constant current set by the current regulator has to be equal to the constant average value of the output current IOUT. If the constant current set by the current regulator is higher than the constant average value of the output current IOUT, the NMOS transistor 14 will be operating in a triode region, causing a ripple current to flow in the LED module 13, and thereby result in a flicker phenomenon. On the other hand, if the constant current set by the current regulator is lower than the constant average value of the output current IOUT, an excess current will flow through the capacitor 12 to lift up the level of the output voltage VOUT, which can result in an overheat hazard of the driver system.
However, as general resistors have a variance in resistance, the current-setting resistor 16 can therefore cause a mismatch between the constant current set by the current regulator and the constant average value of the output current IOUT, and thereby cause the problems mentioned above. To avoid the problems, the current-setting resistor 16 can use a precise resistor. However, the cost will be much higher. Besides, when a variety of different constant average values of the output current IOUT are required (so as to provide a dimming function, for example), the setting of the current regulator will have to be changed accordingly, which can cause much inconvenience to the manufacturer.
To solve the foregoing problems, a novel LED driving method is needed.